Chlorine-based chemicals, such as chlorine, chlorine dioxide, and hypochlorite, have been used in pulp bleaching for several decades, and continue to be used for removing lignin and bleaching the pulp to high brightness. In general terms, the extent of bleaching, hence the degree of brightness obtained, is determined by the type of pulp being bleached and the proposed end use of the paper product. For example, kraft pulps intended for use in making fine writing papers require different bleaching circumstances which will produce the desired brightness in the final paper product. In all instances, however, where chlorine-based bleaching agents have been employed in the prior art, there are produced chlorinated organics. These compounds are generally insoluble and substantial quantities are swept from the pulp with the effluent from one or more of the stages of the bleaching sequence. A relatively smaller percentage of such chlorinated organics remain in the pulp and eventually appear in the paper product.
In chlorine-based bleaching processes, the "C" factor of the pulp is employed as a measure of the chlorination charge required for a specific pulp. By definition, the "C" factor refers to the effective chlorination charge and is equal to the chlorine dioxide plus the chlorine in the charge (expressed in terms of effective chlorine), divided by the Kappa number of the pulp. Generally, it is stated in the prior art that higher "C" factors produce brighter pulps, but that a "C" factor 0.22 provides maximum brightness in a pulp without unacceptable degradation of the cellulosic fibers (e.g. reduced strength). Thus, it has been common heretofore when seeking maximum brightness of the pulp, (in the range of 70-85% GE) to use large "C" factors, i.e. larger quantities of chlorine-containing bleaching agent.
In the past decade, there has been a growing concern about the environmental impact of chlorinated organic compounds in bleach plant effluent. Also, public concern for the disposal of paper containing organically bound chloride has been increasing. Undesirable chlorinated organics such as dioxin have been detected in the exhaust gases of incinerators burning municipal wastes containing chloride, including, for example, paper products made from chlorine bleached pulps. West German environmental regulations, for example, propose restricting the total chloride residue for packaging material including wood pulp to less than 200 ppm. The allowable adsorbed organic halogens (AOX) discharged in the effluent per ton of wood pulp has been restricted to 2.0 kg or less. More stringent regulations are expected in the near future.
Several options have been proposed or practiced to reduce or eliminate chlorinated organics in the bleaching process. The most straightforward method is to substitute non-chlorine bleaching chemicals, such as oxygen, peroxides, ozone, peracetic acid, etc. for chlorine-based bleaching chemicals. Unfortunately, no chlorine-free bleaching process has been developed with the ability to produce acceptable pulp properties (such as brightness or viscosity) at an acceptable bleaching cost.
Another option to reduce the discharge of chlorinated organic compounds is to reduce the chlorine usage in the first stage of the bleaching process. Two alternatives that produce no significant degradation of pulp properties have been commercialized for this purpose. These are (a) extended delignification in the cooking stage and (b) oxygen delignification. These alternatives, with proper extraction, reduce the lignin content of brown stock going into the bleach plant. They do not, however, reduce the chlorinated organic compounds in the bleached pulp and effluent to sufficiently low levels.
A third option to reduce the generation of chlorinated organics in a bleaching process is to substitute chlorine dioxide for chlorine. Chlorine dioxide is a relatively strong oxidant compared to chlorine; to achieve the same degree of delignification, it requires only about thirty-eight weight percent chlorine dioxide on the pulp compared with one hundred weight percent of chlorine. However, these prior art processes are of the DEDED type wherein the chlorination stage (D) is followed by the conventional extraction (E) and additional chlorination (D) stages. The pulp and the discharge effluents resulting from this prior art option contain higher concentrations of chlorinated organics than are acceptable and/or desirable. Processes using both oxygen delignification and chlorine dioxide substitution have been suggested but do not achieve the regulated concentrations of chlorine-containing residues in either the pulp or the effluent.